1. Field of the Invention
This invention relates to pneumatic tires, especially to tires where a carcass is reinforced by a circumferential belt layer comprising reinforcing elements extending in straight, waved, or zigzag manner substantially along the tire circumference.
2. Description of the Prior Art
There are known many ways of strengthening circumferential rigidity of a belt in pneumatic tires by adding a circumferential belt layer to the conventional inclined belt layer having reinforcing elements extending at an angle with respect to the equatorial plane of the tire.
For example, Japanese Patent Laid Open No. 60-22504 discloses a tire having the circumferential belt layer comprising reinforcing elements having a Young's modulus of not less than 2500 kgf/mm.sup.2 whereby the circumferential rigidity is enhanced and separation failure at the belt ends is prevented.
Since the elements of the circumferential belt layer are disposed substantially along the circumferential direction of the tire, the elements are tensioned and lengthened in the circumferential direction in the ground contacting area. In particular, since the diameter of both tread side portions is less than that of the tread center portion, the diameter of both tread side portions is necessarily expanded, so that the reinforcing elements are considerably lengthened. That is, since the reinforcing elements at the side end portions of the circumferential belt layer are lengthened repeatedly in the circumferential direction during rotation of the tire, the reinforcing elements are apt to be broken at the side end portions of the circumferential belt layer. This is a disadvantage of a belt provided with a circumferential belt layer.
An object of the present invention is to prevent the disadvantage caused by providing a circumferential belt layer, in particular breaking of the elements at the edge portions of the circumferential belt layer, and to provide a pneumatic tire having strengthened circumferential rigidity.